1. Field of Invention
This invention relates to a shielding wire for electronic equipment such as copiers, facsimile machines, audio devices, such as tape recorders, and the like, and further for automobiles. The present invention also relates to a shielding tape to be used with a shielding wire, wherein the shielding wire is a flat cable type wire that can improve productivity, without adversely affecting processing terminals of the shielding wire, and can provide excellent shielding of electromagnetic noise. The present invention is also directed to shielding wires using such a shielding tape. Further, this invention also relates to a shielding wire composed of an environmentally-friendly, non-halogen-containing material that does not pollute the environment and does not adversely affect the shielding or production of the wire.
2. Description of Related Art
Flat cable type shielding wires, as shown in FIG. 1, have been used as a shielding wires for electronic equipment including, but not limited to, copiers, facsimile machines and audio equipment.
In flat cable type shielding wires, core wires 3, 3 are formed integrally by wrapping lead wires 1 with insulation sheathing materials 2 (generally polyvinyl chloride) and drain wire 4, arranged in parallel with fixed distances. Also, a shielding tape 5 is adhered closely on the outer peripheries of the core wires 3, 3 and the drain wire 4, while shielding tape 5 is adhered to form bridging parts 7, 7' for maintaining fixed distances. The shielding tape 5 is further protected with sheaths 6 (generally polyvinyl chloride). In the above-described shielding wire, the insulation shielding material 2 and the sheath 6 generally exhibit excellent thermal resistance, insulation and flexibility. In addition, because the shielding material 2 and the sheath 6 are made of polyvinyl chloride, the shielding material 2 and the sheath 6 are relatively inexpensive.
The shielding tape, shown in FIG. 2A, comprises a shielding layer 11, a reinforcing seat layer 12, laminated on the side of a sheath 6 of the shielding layer 11, a sheath adhesive agent layer 13, for adhering the reinforcing seat layer 12 and the sheath, and a core adhesive agent layer 14, for adhering the shielding layer 11 and the core wire 3. The shielding layer 11 is generally composed of a combination of a metal foil 11a of high electric conductivity, such as copper or aluminum, and a metal foil 11b of high permeability, such as iron or tin. Since the reinforcing seat layer 12 exhibits insufficient strength when combined only with the shielding layer 11 composed of the metal foil, the reinforcing seat layer is ordinarily laminated with a polyester in order to increase strength without substantially increasing cost. Although, laminated shielding tapes are preferred, the present invention may also be directed to shielding tapes that are not laminated with a polyester reinforcing seat layer. Generally, the sheath adhesive agent layer 13, includes adhesive agents of the polyethylene terephthalate group (hereafter referred to as "PET"). In addition, the core adhesive agent layer 14, generally includes adhesive agents of a copolymer of vinyl chloride and vinyl acetate (hereafter briefly called a "vinyl chloride-vinyl acetate copolymer").
The core adhesive agent layer 14 is, as shown in FIG. 2B, formed by coating the core adhesive agent in oblique stripes on one side of the shielding layer 11. That is, pails 14a, to be coated with the core adhesive agent, and parts 14b, not to be coated, are alternately arranged in a lengthwise direction of the tape, and in the non coating parts 14b, the metal foils composing the shielding layer 11 are exposed. Thus, via the non-coating parts 14b, the drain wire 4 and the metal foil composing the shielding layer 11 are directly contacted to provide electric conductivity. The shielding tape 5 is thereby grounded to shield electromagnetic waves inside and outside of the cable that can cause strains of signals or noises. On the other hand, by adhesion of the coating part 14a of the core adhesive agent layer 14, the shielding tape 5 closely adheres to the core wire 3 to prevent the core wire 3 from slipping out, and to form bridges 7, 7' between the core wires 3 and 3 as well as between the core wire 3 and the drain wire 4 to prevent misregistration of the core wire 3.
With respect to processing at a terminal of a flat cable type shielding wire having the above-structure, an outermost diameter of the portion of the drain wire 4 (outer diameter shielded with the shielding tape 5 and the sheath 6) is made equal to the outer diameter of the core wire 3, and the terminal is processed using a pressure connection system. The pressure connection system is, as shown in FIG. 3, operates so that the sheath 6 and the shielding tape 5, shielding the core wires 3, are scaled off to expose the core wires 3 and so that a portion of the drain wire 4 is covered with the shielding tape 5 and the sheath 6. Then, under these conditions, contact pins 20 are inserted.
The insertion of the contact pin 20 is performed so that connecting terminals (not shown) having a number of connecting contact pins 20, as shown in FIG. 4A, corresponding to a total number of the core wires 3 and the drain wires 4, are inserted with the exposed core wires 3 (FIG. 4B). In addition, the core wires 3 are pushed so that the contact pins 20 contact the conductors 1 (FIG. 4C), and are further forced into the conductors 1 (FIG. 4D), so that the conductors 1 break through the insulation shielding materials 2 and contact the contact pins 20. In a case of the drain wire 4, the contact pin 20 breaks through the shielding tape 5 and sheath 6, and contacts the drain wire 4.
In such a connection system, by maintaining a fixed pitch between the contact pins 20, 20, a pitch between the core wires 3, 3 and a pitch between the core 3 and the drain wire 4, it is possible to automatically cut the shielding wires, exfoliate the shielding tape 5 and the sheath 6, and automatically attach the connecting terminals by pressure.
Therefore, to accurately connect the conductors 1 of the plurality of core wires 3 in the shielding wires and the contact pins 20 in a bundle, it is necessary to fix the pitch between the core wires 3, 3 and the pitch between the core wires 3 and the drain wire 4 in response to the pitch between the pins 20, 20. For this, the shielding tape 5 should be close to the core wire 3 and the drain wire 4 so as to prevent the core wire from misregistration even after scaling off the shielding tape 5 and the sheath 6.
On the other hand, if the adherence between the shielding tape 5 and the core wire 3 is too great, when exfoliating the sheath 6 and the shielding tape 5 from the terminal of cut face, only the sheath 6 is removed, and the shielding tape 5 remains adhered to the core wire 3. If the shielding tape 5 adheres to the core wire 3, the contact pin 20 and the shielding layer 11 of the shielding tape 5 will likely come in contact, and will result in undesirable contact shorts to the drain wire 4 via the shielding layer 11.
In order to solve these problems, manufacturers have developed shielding wires, wherein the exfoliation of shielding tape is made easier to prevent contact shorts that can occur during processing at the terminals. For example, JP-A-4-133319U gazette discloses a shielding wire wherein a core wire is shielded on its outer periphery with a tape coiled layer coated with a mold releasing agent. Thus, the shielding wire of JP '319 is covered with a shielding tape via a tape coiled layer. By shielding the wire using a tape coiled layer, the shielding tape is easily released from the core wire, between core wires, and between a core wire and a drain wire. Accordingly, the core wire can be prevented from misregistration by the close mutual adherence of shielding tapes. However, in order to shield a wire according to the process of JP '319, an additional step of forming a tape-coiled layer is required. Because JP '319 requires an additional step in the manufacturing process, the overall cost of production is substantially increased.
On the other hand, in order to facilitate exfoliation, manufacturers have proposed making the areas of the coating parts 14a of the core adhesive agent layers 14 smaller. Unfortunately, making the area of the coating part 14b smaller ends up decreasing the adhesive force between the core wire 3 and the shielding tape 5. In addition, the adhesive force between the mutual shielding tapes at the bridge parts 7, 7' is reduced, causing the core wire 3 to experience misregistration and slipping. As a result, this approach is not always a suitable solution.
In conventional shielding tapes, areas connecting between mutual coating parts 14a are made larger than areas between the coating parts 14a and the non coating parts 14b of the core adhesive agent of the shielding tape 5 in the bridge parts 7, 7'. This is done to strengthen fixing of positions of the core wires 3. For this purpose, an area of the coating part 14a is made larger than that of the non coating parts 14b. In short, a width (W1) of the coating part 14a is set to be larger than a width (W2) of the non coating part 14b. Actually, a width (W1) of the coating part 14a is around 2 mm and a width (W2) of the non-coating part 14b is around 1 mm.
As shielding wires for automobiles, collective strands of core wires and drain wires are cylindrically covered with shielding tape. However, because flat cable type wires are capable of omitting a bundle of wires during production and reducing the number of steps needed for processing of shielding wire terminals, manufacturers have considered using flat cable type wires that are similar to public welfare shielding wires used in, for example, audio and electronic equipment. If automobile shielding wires are made using flat cable type wires, flat cable type shielding wires produced for public welfare may be employed. The use of such wires makes it possible to centralize production lines, thereby reducing overall production cost.
However, the recent development of automobile electronics has created an increased demand for more intensive shielding properties. Because modern automotive electrical systems require additional shielding, the flat cable type shielding wires currently available for public welfare cannot be used.
When polymers containing halogen and polyvinyl chloride are exposed to high temperatures (e.g. 80 to 120.degree. C.), poisonous gases, including hydrohalogen gas, are produced.
In order to minimize potential environmental hazards, manufacturers have considered using non-halogen materials, such as polyolefin group polymers, as materials used to make the sheath and the insulation shielding material used in shielding wires. However, when materials of the polyolefin group are used to make the sheath and the insulation shielding material in flat cable typed shielding wires, the balance of the adhesive force of the shielding tape to the sheath and the insulation shielding material is broken. Thus, the likelihood of the resulting wire being an inferior product is substantially increased.
Adhesive agents of the PET group generally used as the sheath adhesive agent do not adhere strongly to adhesive agents of polyolefin group. As a result, only the sheath is scaled off during exfoliation, and shielding tape remains adhered to the surface of the core wire.
In addition, adhesive agents of the vinyl chloride-vinyl acetate group may generate poisonous gases, such as polyvinyl chloride, that cause environmental pollution. Accordingly, there is a need for non-halogen materials that can be used in place of environmentally hazardous adhesive agents.